The objective of this research is to determine the molecular structure of calmodulin, using standard protein crystallographic techniques. This calcium-binding protein, which has been sequenced and has a molecular weight of 16,700, has been crystallized in this laboratory. Three-dimensional x-ray diffractometer data have been collected for the native crystals and four derivatives. Using these data, a three-dimensional electron-density map has been calculated and contoured on acetate sheets. During the next few years, the polypeptide chain will be traced, and then the structure will be refined. The crystal diffracts to high resolution (1.8 Angstrom), so very accurate structural data should be obtainable. Since calmodulin possesses a high degree of sequence homology to troponin-C, parvalbumin, and vitamin D-dependent calcium binding protein, its structure will provide information about this general class of calcium-binding proteins. In addition, the structure should provide clues to its function as a regulator of numerous enzymes.